Correct Word Use

This is a glossary of lots of words which are commonly misused.
http://grammar.about.com/od/words/a/UsageGlossary.htm
The list is HUGE and you can look into as many as you like, although I particularly want you to know how to use the following:

accept/except/expect

advice/advise

are/our

desert/dessert

if/whether

last/later/latter

many/much

quotation/quote

shall/will

should/would

who/whom

This website has a huge list of quizzes! It is aimed at adult english as a second language learners, but I think you may find them useful. Scroll down to 'Confusing Word Pairs' to find the ones that will be relevant.
http://grammar.about.com/od/words/a/UsageGlossary.htm

A fill in the blanks exercise for should/would/can/could:
http://www.englishpage.com/modals/interactivemodal1.htm

This quiz is quite difficult in places, but you may like to have a go anyway - perhaps with a dictionary in hand! Only for the VERY brave!
http://www.quizmoz.com/quizzes/English-Language-Quizzes/e/English-Word-Usage-Quiz.asp